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Anthrax. Smallpox. A potentially deadly strain of the flu virus. The American public trusts the Centers for Disease Control and Prevention to handle dangerous biological materials, but recent incidents have put a damper on the agency's reputation, members of Congress said Wednesday.

"What happened was completely unacceptable," CDC Director Dr. Tom Frieden said in his testimony before the Energy and Commerce Committee's subcommittee for Oversight and Investigations.

"It never should have happened."

Rep. Tim Murphy, R-Pennsylvania, the subcommittee chairman, opened the hearing by summarizing the troubling incidents that have come to light over the past month.

In early June, dozens of CDC workers were potentially exposed to anthrax after a lab failed to inactivate the dangerous bacteria before transferring it to a lower lab.

A subsequent investigation revealed "an alarming series of failures," according to one of the subcommittee members, including an incident of cross contamination between a relatively safe and a potentially deadly strain of the flu virus.

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An outside investigation by the U.S. Department of Agriculture found dangerous biological materials stored in unlocked refrigerators and a general lack of lab workers following safety protocols. Investigators say the anthrax that was believed to be deactivated was transferred in Ziploc bags, which are obviously not approved to carry such materials.

"This is troubling, and it is completely unacceptable," Murphy said. "The CDC is supposed to be the gold standard (for lab safety). ...This is not sound science, and this will not be tolerated. These practices put the health of the American public at risk."

Murphy said the CDC was lucky that no one appears to have been infected during the anthrax incident, but "sooner or later that luck will run out."

This is not the first time the subcommittee has heard of safety issues at CDC labs, said Rep. Diana DeGette, D-Colorado. Multiple hearings over the past decade have shown lapses that together reveal a problem with safety reporting at the agency, she said.

"What we all need to know is the plan to change the culture at the CDC," DeGette said. "We can do a lot but we can't legislate a culture change."

Frieden, who took the CDC director job in 2009, acknowledged that he and other CDC managers failed to recognize a "critical pattern."

The CDC has closed the labs that were involved in the incidents. They won't reopen until better safety procedures are in place, Frieden said. Scientists have also stopped moving any biological material out of its two highest-level labs while their procedures are being re-evaluated.

This may slow some CDC processes -- such as providing materials for companies to create next year's flu vaccine, Frieden said -- but safety comes first.